1. Field of the Invention
The present invention relates to a burst signal receiving apparatus, and more particularly, to a burst signal receiving apparatus in a optical digital transmission system.
2. Description of Related Art
FIG. 1 is a block diagram illustrating a conventional burst signal receiving apparatus in an optical digital transmission system. An optical digital burst signal 122 is supplied from an optical transmission line (not shown) of a station via an input terminal 120. The optical digital burst signal 122 is composed of frames carrying data transmitted from other stations. Each of the frames of fixed length includes a plurality of bits and is composed of a head bit pattern, a sync bit pattern and a data bit pattern indicative of the data. The optical digital burst signal 122 is converted into an electric digital burst signal 124 by an optical signal--electric signal (O/E) converter 111. An automatic threshold control (ATC) circuit 116 receives the electric digital burst signal 124 from the O/E converter 112 to detect a peak level and bottom level of the burst signal 124 and calculates an average value of the peak level and the bottom level to output to an identifying circuit 112 as a DC threshold voltage 126. The identifying circuit 112 receives the electric digital burst signal 124 from the O/E converter 111 and the DC threshold voltage 126 from the ATC circuit 116 and uses the threshold voltage 126 to identify the level of each of bits of the burst signal 124 to produce a digital burst signal 128 which is outputted to an output terminal 130. A clock signal generating circuit 113 generates a clock signal 132 in synchronous with the digital burst signal 128. The clock signal 132 is used in the apparatus and supplied to a synchronization detecting circuit 114 and a counter 115. The synchronization detecting circuit 114 detects a sync pattern of the burst signal from the digital burst signal 128 outputted from the identifying circuit 112 in synchronous with the clock signal 132 to output the detecting result to an output terminal 140 as a sync information 136. Also, the detecting circuit 114 outputs a hunting information 134 indicative of establishment of synchronization to the counter 115 if the sync pattern is once detected. The counter 115 is initialized in response to the sync information 136 and counts the clock pulses of clock signal 132 to output the count to an output terminal 150 as a count information.
More particularly, in a case where an optical digital communication system includes a parent station and a plurality of child stations and one way transmission is carried out between one parent station and a plurality of child stations via a star coupler, a downstream signal transmitted from the parent station to the plurality of child stations is transmitted in serial intermittently with a predetermined time interval. The burst signal is received by the burst signal receiving apparatus in each of the plurality of child stations. Because transmission path lengths from the parent station to the plurality of child stations are different from each other, the received signal level of the burst signal in one child station are different from that in another child station by a difference in transmission loss due to the difference in the transmission path length. The components of the burst signal receiving apparatus shown in FIG. 1 operate in the same manner in each child station. That is, the identifying circuit 112 is required to reliably identify the "1" or "0" level of each bit of the digital burst signal 124 even if the burst signal is transmitted though a long optical transmission path after the star coupler. For this reason, the ATC circuit 116 detects the peak level and bottom level of the digital burst signal 124 to automatically determine the threshold voltage 126 as the average voltage of these levels. For instance, if the threshold voltage 126 is low, a noise level is erroneously detected as the "1" level of the burst signal. On the other hand, if the threshold voltage is high, the peak level of the burst signal 124 transmitted from the child station having the longest transmission path length from the parent station is lower than the threshold voltage, so that it would be impossible to identify the high signal level of the burst signal 124 in the farthest child station. Accordingly, the threshold voltage level is actually determined to satisfy the above two conditions. Further, transmission loss of the transmission path changes as temperature change other than the difference of transmission path length and the signal level of the burst signal also changes due to the transmission loss. However, since change of seasons is a major factor in the transmission loss change of the transmission path by temperature change, the signal level change of the burst signal is so gentle that it is readily possible to automatically change the threshold voltage in accordance with the signal level change of the burst signal.
In the conventional burst signal receiving apparatus, the signal level identification of the burst signal can be easily performed in each of the plurality of child stations in a case of one way transmission from the parent station to the plurality of child stations. However, in a case of bidirectional transmission between the parent station and the plurality of child stations, the signal levels of the burst signals transmitted from the plurality of child stations to the parent station are different from each other by the transmission loss differences due to the differences of the transmission path lengths. In the worst case, there would be a case where a burst signal having the lowest peak level is received via the longest transmission path just after a burst signal having the highest peak level is received via the shortest transmission path. In such a case, since the threshold voltage is kept at a value determined based on the relatively high peak level, when the burst signal having the relatively low peak level is suddenly received, the burst signal receiving apparatus cannot track the level change in the peak level of the burst signal. If the threshold voltage is higher than the relatively low peak level of the burst signal, the identifying circuit 112 cannot identify the signal level of "1" of the burst signal having the relatively low peak level.